1. Field of the Invention
The present invention relates to a mold-clamping control method for an injection molding machine, which method is favorably used in detecting a foreign object that is caught between a movable mold and a stationary mold during the course of a mold clamping step.
2. Description of the Related Art
Conventionally, in the field of an injection molding machine designed such that reciprocating motion produced by a drive unit composed of a servomotor and a ball-screw mechanism is transmitted to a movable platen via a toggle link mechanism, there has been known a foreign-object detection method for detecting a foreign object (e.g., a molded product) that is caught between a movable mold and a stationary mold upon movement of the movable platen in a mold closing direction (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-172670).
The patent publication discloses a foreign-object detection method for an injection molding machine, including the steps of detecting a physical value related to a mold closing action in a monitor region during the course of a mold clamping step; and performing emergency processing, such as stopping of a servomotor, a retreat operation, and generation of an alarm, when a deviation between the detected physical value and a predetermined theoretical value is in excess of a threshold value. Notably, the threshold value is determined as follows: a trial mold clamping operation is performed to thereby detect a maximum value of the deviation; and the maximum value is added to a preset reference value to thereby determine a threshold value to be used for foreign object detection.
In the foreign-object detection method (mold-clamping control method), a physical value related to a mold closing action in a monitor region is detected; and when a deviation between the detected physical value and the predetermined theoretical value is in excess of the preset threshold value, emergency processing is performed. Therefore, a time lag arises before emergency processing is performed after a foreign object (a molded object) is caught between a movable mold and a stationary mold. A long time lag causes various drawbacks; for example, accurate monitoring of a molded product cannot be performed because of breakage of a molded product, or a mold may be damaged. Therefore, shortening the time lag to the greatest possible extent is preferred.
However, in a conventional foreign-object detection method, after a servomotor is caused to stop operating, a retreat operation is performed under ordinary mold-opening control; thus, an unignorably long time lag is involved. As a result, the conventional method fails to reliably avoid occurrence of, for example, breakage of a molded product and damage to a mold.